What is the difference between the Field Stress and Body Force options?

Here is a tip taken from the Initial Element Loading section of the RS2 Online Help.

Users often are confused by the concept of Initial Element Loading, and how it relates to the Field Stress defined in the Loading menu. Here are some further comments:

The initial element loading is one of the most difficult concepts to grasp in Finite-Element (FE) modeling. Basically, in FE an element can have two initial internal loadings, initial stress and body force. Body force is just self weight. If you use just body force in a model then you will notice that a component of the displacements is due to your model settling under it's own weight. If you use materials with initial element loading set to Body Force Only then the Field Stress defined in the Loading menu is NOT used at all. This brings us to the case of initial stress. An initial stress locks in a particular stress in an element. The stress that is locked into the element is defined in the Field Stress dialog. Think of a finite element as a sponge, applying an initial stress is like compressing the sponge. If you release confinement on an edge of the sponge it will expand in that direction. This is basically what happens when you open up an excavation in a material with an initial stress. Now how do body force and initial stress relate. They work to balance each other out. Think of an element with initial stress, then put a layer of material on top of it such that the material has enough body force to balance the initial stress. As a result the element is in equilibrium and won't expand or contract. As an example, let's think of a bucket (a rectangular region) of material represented by finite elements. The boundary conditions are free on the top surface and rollers on the left/right/bottom sides. If the material has just body force then it settles under it's own weight and the top surface moves down. If the material has just initial stress then it expands and the top surface moves up. If you have both defined then the material is in equilibrium and there is no displacement of the top surface. To have the body force and field stress balanced you must use a gravitational Field Stress with unit weight equal to the material's unit weight and a ground surface elevation equal to the top of the bucket.

To summarize, if you define Body Force Only, basically the material settles under its own weight. If the material settles under its own weight, the horizontal stress is due to the Poisson effect, thus its value is equal to pr/(1-pr), where pr is the Poisson ratio. If you want to specify a horizontal stress ratio, then you need to use initial stresses.